TABLE OF CONTENTS The GPS was developed by the U.S. Department of Defense to provide precise estimates of PVT to users worldwide [1, 2]. Although the system was initially designed to serve an expected total of 40,000 military users, nowadays the GPS is the mostly widely used navigation system, with more than 20 million civil and military users worldwide.
The growing importance of GPS in both the military and civilian sectors and the need to improve its performance and capability to support high accuracy and high reliability civilian and military services have driven the U.S. government to design a modernization program that will culminate in the implementation of a new navigation system architecture, referred to as GPS III [3 6].
On January 25, 1999, an official release by the U.S. Vice President Al Gore announced a $400 million GPS modernization program [7]. The program s objective is to introduce technological improvements within both the space and control segments and to add new navigation signals to future GPS satellites to improve the performance available for existing services and enable new services in the military, civil, commercial, and scientific sectors worldwide:
The United States is proud to be a leader in the development of the Global Positioning System a wonderful example of how technology is benefiting our citizens and people around the world. This initiative represents a major milestone in the evolution of GPS as a global information utility and will help us realize the full benefits of this technology in the next millennium.
Vice President...
